Internet protocol (IP) multimedia subsystem (IMS) is an architectural framework for delivering application level IP multimedia services such as voice and video over a packet-switched network. An IMS core network layer typically comprises a number of Session Initiation Protocol (SIP) servers and proxies arranged to perform various sub-functions in a call session control function (CSCF) to process SIP signaling packets in the IMS. Those functions include a proxy CSCF (P-CSCF), a serving CSCF (S-CSCF) and an interrogating CSCF (I-CSCF).
SIP servers perform application level switching in layers 4-7 of the seven-layer OSI model of computer networking. That SIP layer 4-7 switching is distinct from SDN layer 2-3 switching that is performed to determine a route through the network between an origination network address and a termination network address. In SDN layer 2-3 switching, a controller collects paths from multiple SDN switches to get a network view, and send instructions to the switches (i.e., match-action table rules) that define routes between the origination and the termination. The routing can be done, for example, based on MAC addresses or IP addresses.
SIP servers performing layer 4-7 switching, in contrast, perform different functions. Specifically, given a message from a URI representing an identity as an origination, given the URI representing an identity for a termination and given a service definition, a SIP server performing layer 4-7 switching determines the actual network address(es) to which to deliver the message, and which functions need to be incorporated in the path. That is done by processing, at each switch, service rules defining edits to the incoming message and selection of the next destination. Multiple legs are used to incorporate functions or gateways rather than to create a routing path. Each leg's routing, once the destination address is determined, may be done using SDN.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.